The need for devices to effectively treat and dispose of waste materials without creating undue environmental contamination has been recognized for many years. Incinerators have been used to reduce municipal solid wastes to ash which must then be disposed of by conventional means, such as burial in a land fill. Incinerators have also been either used for, or proposed for, the disposal of other forms of commonly occurring waste products, such as medical wastes, low level nuclear wastes, hazardous organic or inorganic chemical wastes, etc.
For example, U.S. Pat. No 4,299,611 discloses an incinerator for converting hazardous materials to a relatively harmless condition. Feed material, at least a portion of which is combustible, is directed into a conversion chamber and onto glass which is molten in the lower portion of the chamber. The waste is broken down into an ash component and a gaseous component. A portion of the ash settles into the molten glass, which is discharged to a cooling area to form a solid waste-glass product. Metallic portions of the waste are oxidized to metal oxides and then incorporated into the molten glass.
A number of other incinerator-type devices have been proposed to dispose of asbestos waste. In U.S. Pat. No. 4,820,328, asbestos is converted into a glass by mixing the asbestos with cullet and a melt accelerator, and introducing the mixture onto a quantity of molten glass in a furnace, thereby decomposing the asbestos. The molten glass/decomposed asbestos is withdrawn from the furnace at about 1000 degrees C. Likewise, in U.S. Pat. No. 4,678,493, asbestos is converted into a vitreous glass by introducing the asbestos into a two-chambered furnace. Glass is made from feed materials and cullet in a two-chambered furnace that is both electrically and gas fired in U.S. Pat. No. 4,831,633. Finally, an advanceable tin oxide electrode which can be advanced into additional sections of the furnace as the electrode advances so that the whole of the electrode material needed in a furnace campaign does not have to be present at the outset of the campaign, is disclosed in U.S. Pat. No. 4,897,853.
German Patentschrift DE 41 13 182 C 1 discloses a furnace for vitrification of solid wastes, especially combustion ash and asbestos. A refractory-lined vessel is covered with a hood having a feed pipe therethrough, and product glass is apparently continuously withdrawn from an outlet. French Patent No. 1,320,921 discloses an in situ vitrification process for remediating radioactive wastes. A mixture of radioactive waste and glass constituents is deposited in an excavated area and then vitrified. The vitrified mass is then cooled in place rather than being continuously withdrawn.
Waste materials for input into incinerators or other treatment means may take many varied forms. Such wastes may be combustible or noncombustible, solid or liquid. Regardless of the form of the waste upon entry into the treatment means, it must be converted into a form that can be used or disposed of without environmental insult. Depending upon the chemical constituents in the wastes, they must either be converted to harmless by-products, such as carbon dioxide or water, or alternatively, they must be immobilized (preferably in a much reduced volume) for permanent disposal or storage.
One source of noxious waste is the ash resulting from the incineration of municipal solid wastes (MSW ash). About 180 million tons of municipal solid waste is generated each year in the United States-about four pounds per person per day. About 13% is recycled, 73% is disposed of in land fills and 13% is incinerated, to yield about 10 million tons of MSW ash per year. Projections indicate that by the turn of the century up to 25% of MSW will be incinerated. The ashes sometimes contain potentially hazardous organics and heavy metals which can be leached into ground water. Typical disposal practices for these ashes are conventional land filling or disposal in a special ash disposal site.
It has been proposed that certain wastes can advantageously be melted, or vitrified, to reduce or eliminate the toxicity and disposal concerns. However, prior art melters suffer from high capital costs due to costly refractory materials that must be used. Generally speaking, fused cast refractory melters are held in position by a mechanical frame that must be capable of accommodating large thermal expansion. Adding to the expense of the device, the refractory material typically "wears out" after about 3 years of use. The precipitation and accumulation of metals on the refractory floor, resulting from the highly reducing conditions within the melter or from the content of metal in the feed, can result in accelerated failure of refractory materials by the phenomena known as "downward-drilling".
Because of these and other problems incident to current refractory lined melters, Applicant's invention represents a significant step forward in this art. This invention is relatively low cost and long lived, with a simple design allowing for easy construction and operation. Due to the design, the constraints of such a facility will be substantially less than with state-of-the-art refractory melter designs. In view of the foregoing, it is an object of the present invention to provide a method and apparatus for converting feed materials to a consolidated, relatively harmless melted condition, suitable to alternative productive uses or for long-term storage without environmental insult.
It is a further object of the invention to provide an apparatus that is easy and inexpensive to build, whether as a below-grade excavation, as a stand-alone device manufactured for above-ground siting, or a mobile truck-transportable device, and which will convert feed materials to a glass- or rock-like material having the hazardous constituents in the feed materials either destroyed or chemically bound in the discharged material.
These and other objects will become apparent from the following description.